CHEMISTRY OF ALKENES

1. CHEMISTRY OF ALKENES

Introduction

Molecules of the alkene (also called olefins) series of hydrocarbons are characterized by having two adjacent carbon atoms joined to one another by a double bond. The carbon-carbon double bond is unsaturated and hence highly reactive toward a wide variety of reagents. The general formula of alkenes is CnH2n , where n is the number of carbon atoms.

Examples: Ethene, Propene, Butene, Pentene etc.

Isomerism in Alkenes

Isomeric compounds are also possible in the alkenes. For the molecular formula C4H8, there are three different ways of organizing the four carbon atoms and the double bond.

 

Physical Properties of Alkenes

The alkenes possess physical properties that are essentially the same as those of the alkanes. They are insoluble in water, but quite soluble in non-polar solvents like benzene, ether, and chloroform. They are less dense than water. The boiling point rises with increasing carbon number; as with alkane, branching lowers the boiling point. Like alkanes, alkenes are at most only weakly polar. Since the loosely held π electrons of the double bond are easily pulled or pushed, dipole moments are larger than for alkanes.

 

Chemical Properties of Alkenes

The presence of the carbon-carbon double bond confers very considerable chemical activity on the alkenes and consequently they react with a much wider variety of reagents than do the alkanes. The π-bond component of the double bond is weaker than the σ-bond component and the π-electrons are more easily accessible to attacking reagents.

 

1. Addition of hydrogen

When an alkene is mixed with hydrogen, there is no appreciable reaction, but in the presence of certain metal catalysts such as nickel, platinum or palladium, a fairly rapid reaction occurs leading to the uptake of hydrogen.

 

of the double bond is weaker than the σ-bond component and the π-electrons are more easily

 

2. Addition of halogen

Chlorine and bromine react readily with alkenes, in the liquid or vapour states, to form dihalogeno addition products, the two halogen atoms are attached to adjacent carbons. Iodine generally fails to react.

 

 

 

3. Addition of hydrogen halides

Alkenes react, usually on heating, with hydrogen halides (either gaseousor in concentrated solution) to form an addition product.

 

The hydrogen attaches itself to the carbon that already holds the greater number of hydrogens. This statement is called Markovnikov's rule.

 

If the reaction takes place in the absence of peroxide, the addition of HBr, to alkenes follows Markovnikov's rule. On the other hand, if the reaction takes place in the presence of peroxide, HBr adds to alkenes in the reverse direction.

 

 

 

4. Addition of water

It is made to react with water in the presence of few drops of sulphuric acid. This results in the formation of alcohols as per Markovnikov rule.

 

Preparation of Alkenes

1. Reduction of alkynes

Trans alkene can be obtained by reduction of alkynes with sodium or lithium in liquid ammonia. Cis alkene is obtained by hydrogenation of alkynes with a specially prepared palladium called Lindlar catalyst.

 

2. Dehydrohalogenation of alkyl halides

Dehydrohalogenation involves loss of the halogen atom and of hydrogen atom from a carbon adjacent to the one losing the halogen (1,2- elimination).

 

 

3. Dehalogenation of vicinal dihalides

Alkenes can be prepared by elimination of two halide atoms from a vicinal (neighboring) dihalides.

 

 

4. Dehydration of alcohols

In the dehydration of alcohols, the H and OH are lost from adjacent carbons (1,2-elimination). An acid catalyst is necessary and application of heat.

 

 

Uses of Alkenes

• Lower individuals from alkenes are utilized as energizers and illuminants.
• Alkenes and subbed alkenes upon polymerization from numerous valuable mixtures like polythene, PVC, Teflon, orlon, and so on
• Alkenes are utilized for the counterfeit maturing of organic products.
• Ethylene is utilized in the assembling of ethyl liquor and ethylene glycol.
• Alkenes are utilized for making hostile to thump for motors.
• Ethylene is utilized in making oxygen-ethylene fire for cutting and welding